1. Field of the Invention
This invention relates to a method for forming a deposition film, which is suitable for formation of deposition films, above all functional films, particularly amorphous or crystalline deposition films for constituting semiconductor devices, photosensitive devices for electrophotography, line sensors for image inputting, image pickup devices, photovoltaic elements, etc.
2. Description of the Prior Art
The optical CVD method, in which a gas containing a depositing material element for a deposition film is decomposed photochemically by irradiation of a light energy such as UV-ray to form a deposition film on a substrate by use of molecular or atomic active species formed by dissociation of the gas, has been known as a technique for forming a deposition film at a low temperature.
For example, as the optical CVD method for forming silicon films of non-doped or doped hydrogenated amorphous silicon, etc., there have been attempted the methods in which silane gas (SiH.sub.4) is decomposed directly with excimer laser of, for example, argon fluoride (ArF), krypton fluoride (KrF), etc.
Whereas, such optical CVD methods of the prior art involve some problems in practical application, above all two important problems that (1) a film is deposited on the window through which a light is introduced into the reaction chamber where deposition of a film is to be effected on a substrate, whereby the light cannot reach the substrate and that (2) light absorption cannot efficiently be done even if the light may reach the substrate.
As a measure against the problem (1) among these problems, it is possible to use the method, as reported in Spring Season Meeting of Society of Applied Physics of Japan, 1984, in which ArF excimer laser with longer wavelength (wavelength 193 nm) than the upper limit of absorbed wavelength of silane 170 nm (corresponding to 7.3 eV) is used as the light source, thereby enhancing dramatically the light intensity in the vicinity of the substrate relative to the vicinity of the window. According to this method, the so called two photon absorption reaction, in which a silane molecule absorbs two photons of ArF excimer laser (corresponding to 6.4 V) at the same time, will become prevailing, while the one photon absorption reaction will become prevailing in the vicinity of the window. As the result, of the reactions which dissocisate silane molecules to form active species such as SiH*, SiH.sub.2 *, Si*, H*, etc , the activation energy 7.3 eV necessary for forming the active species SiH.sub.2 * according to the following reaction giving the threshold value: EQU SiH.sub.4 .fwdarw.SiH.sub.2 *+H.sub.2
is surpassed only in the case of the two photon absorption, namely the reaction in the vicinity of the substrate, and no silicon film will be deposited on the window.
However, according to this method of laser two photon absorption, the above problem (2) of light absorption efficiency cannot be solved. Shortly speaking, in the case of two photon absorption by laser, if excitation energy levels of the gas containing film depositing material element such as SiH.sub.4 are expressed by E.sub.n (n=1, 2, 3 . . . ) and the photon energy of laser are expressed by E.sub.p, resonance absorption will occur only when: EQU E.sub.n =2E.sub.p,
and the light absorption efficiency is low at other energy levels.
More specifically, as schematically shown in FIG. 2, if the excitation energy levels of the gas containing film depositing material element are expressed as E.sub.1, E.sub.2, E.sub.3, E.sub.4, etc. in the order of energy required for the activation reaction from the lower energy side, for example, when the energy E.sub.p of laser was selected in conformity with the Level of E.sub.3 to exite two photon absorption, the absorption efficiency was low in the case of 2E.sub.p .noteq.E.sub.3 to give no sufficient deposition speed. This is the same when 2E.sub.p was conformed to any of E.sub.1, E.sub.2 and E.sub.4.
Accordingly, one may consider to excite two photon absorption by use of a continuous polychromatic light such as Xe lamp in place of laser which is monochromatic light. However, in this case, by use only of a Xe lamp, which is low in luminance of the light source, no satisfactory deposition speed can be obtained.